Microwave-generated plasmas are used in many deposition, etching and substrate processing operations. Microwave plasma reactors typically include a vacuum chamber containing a gas to be energized to form the plasma. Microwave energy is introduced to the chamber through a dielectric window or dielectric barrier to maintain the vacuum in the processing chamber while providing a means of allowing the microwave energy to enter the chamber. The vacuum chamber may include the microwave cavity or the vacuum chamber and cavity may be separate volumes that may intersect or overlap.
Such microwave devices, however, suffer from a number of problems which have kept them from being widely accepted for industrial processing applications. The need to carefully place and seal the dielectric window creates difficult to disassemble devices that do not lend themselves to the easy substrate access required for industrial applications in which speed and ease is critical. Further, the dielectric window is exposed to the plasma. Since the windows cannot be subjected to the high temperature levels required for plasma operation, it is necessary to design the chambers so that the plasma does not touch the window. However, this limits the amount of power that can be coupled to the plasma, because plasma size is directly related to power for a given pressure. Further, the plasma tends to etch the window, thereby slowly destroying it. Additionally, this etching causes contamination of the chamber and the substrate with the dielectric window material. Also, the plasma may deposit a deleterious coating on the window which may limit its useful life.
These microwave plasma generators also require that the plasma vacuum chamber be relatively small so that a plasma may be formed with available power. Since plasma temperatures may reach 1000.degree. C. or more, the heat load on the chamber walls is extremely high, requiring complex cooling systems that make the devices more expensive and even more difficult to disassemble and assemble for access to the substrate. Additionally, the plasmas formed are typically uneven (nonsymmetric), causing uneven substrate processing without the addition of some means for increasing uniformity, such as a substrate turntable that continuously moves the substrate in relation to the plasma.
Because of the problems associated with microwave plasma generation, plasma processes such as etching and deposition of materials in semiconductor processing often employ a radio frequency plasma discharge sustained in a parallel plate reactor. A typical RF plasma generator is shown in FIG. 1. Reactor 10 includes chamber 12 having therein parallel plate electrodes 16 and 17. Plate 17 is electrically connected to chamber 12 which itself is on one side of RF power source 22, and the other side of RF power source 22 is electrically connected to plate 16. There is a gas inlet and gas exhaust. Plasma 20 is formed between plates 16 and 17 so that substrate 14 may be processed by plasma 20.
These RF plasma generators typically operate at frequencies of 100 KHz to 13 MHz. The reactors are much smaller in every dimension than the wavelength at the frequency of operation. For example, the wavelength at the upper range of 13 MHz is approximately 20 meters, and the electrode diameter in such reactors commonly is 1/2 meter or less. Additionally, the distance between the electrodes and the height and diameter of the chamber are also much less than a wavelength at the frequency of operation. Because the dimensions of these RF generators are small compared to the length of a standing wave, standing waves and their associated field distributions do not play a significant role in operation of the reactor. Because of the relative low frequency and the size limitations, however, these reactors are relatively low energy and create a relatively low density plasma which is in many instances inferior to the high density microwave plasma with regard to rate, feature definition, and damage levels. Accordingly, although RF plasma reactors are widely used and are less complex in design than traditional microwave plasma reactors and thus relatively simple to operate, they have not been able to keep up with the processing needs of the industry.